Conventional exposure apparatus for manufacturing semiconductor devices include an illumination system for illuminating a circuit pattern formed on a mask and projecting this pattern through a projection optical system onto a photosensitive substrate (e.g., a wafer) coated with photosensitive material (e.g., photoresist). One type of projection optical system employs an off-axis field (e.g., an arcuate field) and projects and transfers only a portion of the mask circuit pattern onto the wafer if the exposure were static. An exemplary projection optical system having such a field comprises two reflecting mirrors, a concave mirror and a convex mirror. In such projection optical systems, transfer of the entire mask circuit pattern onto the wafer is performed dynamically by simultaneously scanning the mask and wafer in a fixed direction.
Scanning exposure has the advantage in that a high resolving power is obtained with a comparatively high throughput. In scanning-type exposure apparatus, an illumination system capable of uniformly illuminating with a fixed numerical aperture (NA) the entire arcuate field on the mask is highly desirable. Such an illumination system is disclosed in Japanese Patent Application Kokai No. Sho 60-232552. With reference to FIG. 1, an illumination system 10, disclosed therein, comprises, along an optical axis A, an ultrahigh-pressure mercury lamp 12, an elliptical mirror 14, and an optical integrator 16. With reference now also to FIG. 2, optical integrator 16 has an incident surface 16i, an exit surface 16e, and comprises a combination of four segmented cylindrical lenses 16a-16d. Lenses 16a and 16d are located at the respective ends of optical integrator 16, are oriented in the same direction, and have a focal length f1. Lenses 16b and 16c are located between lenses 16a and 16d and are each oriented in the same direction, which is substantially perpendicular to the orientation of lenses 16a and 16d. 
Adjacent optical integrator 16 is a first condenser optical system 18 and a slit plate 20. With reference now also to FIG. 3, the latter includes an arcuate aperture 20A having a width 20W and a cord 20C. Adjacent slit plate 20 is a condenser optical system 22 and a mask 24.
Mercury lamp 12 generates a light beam 26 which is condensed by elliptical mirror 14 onto incident surface 16i of optical integrator 16. By virtue of having two different focal lengths, optical integrator 16 causes light beam 26, passing therethrough, to have different numerical apertures in orthogonal directions to the beam (e.g., in the plane and out of the plane of the paper, as viewed in FIG. 1). Light beam 26 is then condensed by condenser optical system 18 and illuminates slit plate 20 and arcuate aperture 20A. Light beam 26 then passes therethrough and is incident condenser optical system 22, which condenses the light beam to uniformly illuminate a portion of mask 24.
With continuing reference to FIG. 3, a rectangular-shaped region 28 on slit plate 20 is illuminated so that at least arcuate aperture 20A is irradiated. Thus, light beam 26 is transformed from a rectangular cross-section beam to an arcuate illumination beam, corresponding to aperture 20A. Note that aperture 20A passes only a small part of the beam incident slit plate 20.
Generally, arcuate cord 20C is made long to increase the size of the exposure field on the wafer. In addition, arcuate slit width 20W is set comparatively narrow to correspond to the corrected region of the projection optical system used in combination with illumination system 10. The illumination efficiency is determined by the ratio of surface area of arcuate aperture 20A to rectangular-shaped region 28. This ratio is small for illumination system 10, an indication that the system is very inefficient, which is disadvantageous. As a result, the amount of light reaching mask 24 is fixed at a relatively low level. Since the time of exposure of mask 24 is inversely proportional to the amount of light (i.e., intensity) at the mask (i.e., the more intense the light, the shorter the exposure time), the scanning speed of the mask is limited. This limits the exposure apparatus' ability to process an increasingly large number of wafers (e.g., to increase throughput).